Field emission is a quantum-mechanical phenomenon whereby electrons in a solid tunnel through the energy barrier at the emitter/vacuum interface and are emitted into vacuum under the influence of an electric field. The energy barrier sharpens and the probability of emission increases as the applied voltage between the cathode and an anode or gate electrode increases. For traditional field emitter materials such as silicon, the electron affinity .chi. (i.e., the difference between the minimum energy of an electron in vacuum and the conduction band edge), is positive. Electrons in such positive electron affinity materials can tunnel through the barrier at the emitter/vacuum junction with practicable probability only at high electric fields (.about.3.times.10.sup.9 V/m). Operation of field emitters, also called cold cathodes, made of these materials requires high applied voltages and local enhancement of the field, such as by an emission tip having small radius of curvature. Furthermore, these field-enhancing geometries render the emitter vulnerable to damage by ion bombardment.
Diamond has been recognized as the most propitious candidate for solid-state field emitters owing to its negative electron affinity ("NEA") under certain conditions (see, e.g., Geis et al., IEEE Electron Device Letters, EDL-12, 456-9 [1991]). The NEA exhibited by diamond should be useful for the fabrication of robust field-emission cathodes that operate at low applied fields and without requiring small-radius-of-curvature structures. Although diamond is not the only known NEA material, it is unique in its possession of other attributes desirable for cold cathodes: low chemical reactivity, low chemical sensitivity, high melting temperature, high thermal conductivity, and robust NEA.
Several field-emission devices using diamond cathodes have been described in the literature. These often rely on so-called Spindt geometry to achieve significant emission. Typically the emitter material is part of a film fabricated using chemical vapor deposition ("CVD") methods. Other methods of incorporating diamond which avoid the high cost and slow growth rate of the CVD diamond synthesis process have also been investigated. U.S. Pat. No. 5,608,283 describes the use of carbon-containing particles, including diamond grit, in the formation of emitter structures. U.S. Pat. Nos. 5,252,833 and 5,278,475 describe cold emitters which include a plurality of diamond crystallites that form a layer of polycrystalline diamond. The origin of the diamond material is not addressed by these two disclosures.
Notwithstanding the great appeal of diamond materials for these applications, the achievement of low-voltage, high-current-density cold diamond cathodes has proven to be elusive. The reason for this failure can be better understood with resort to the energy levels in the diamond bandgap.
FIG. 1 is an energy level diagram for an interface between (111)-oriented diamond and vacuum. The bottom of the diamond conduction band E.sub.C is about 0.7 eV above the vacuum electron energy level E.sub.VAC (see, e.g., Geis et al., IEEE Transactions on Electron Devices, 38(3), 619-626 [1991]). With minimal applied field, electrons in the conduction band could be emitted to vacuum. In undoped diamond, conduction-band electrons are created by thermal activation of an electron from the valence band across the 5.5-eV bandgap. A shallow donor dopant could also be ionized to populate the conduction band and provide a source of electrons for emission. Unfortunately, in current practice no such donors are known, although several elements have been explored as dopants in CVD diamond. Substitutional phosphorous and interstitial sodium occupy shallow donor levels but are not significantly soluble in those positions. Ion-implanted interstitial lithium can act as a shallow donor, but the configuration is not robust to annealing; indeed, lithium-doped diamonds are usually p-type (see, e.g., Geis et al., IEEE Transactions on Electron Devices, 38(3), 619-626 [1991]; Bernholc et al., SiC, Natural and Synthetic Diamond and Related Materials, Proceedings of Symposium C of the 1990 E-MRS Fall Conference, 265-272 [1990]). Nitrogen has proven difficult to incorporate into CVD diamond; the introduction of nitrogen into the process results in a degradation of the diamond crystal structure. In type Ia natural diamond, nitrogen exists in aggregates that form deep donor levels far below the vacuum energy level. In type Ib, high-pressure synthetic diamond, donor nitrogen occupies substitutional positions about 1.7 eV below the conduction band; this position represents the highest stable dopant energy level known in diamond.
However, since the highest donor energy level in diamond is 1 eV below the vacuum level, significant applied fields are still required to allow field emission, notwithstanding the favorable position of the conduction band edge. The lack of practical shallow n-type dopants has been an obstacle to taking advantage of the NEA properties of diamond (see, e.g., Koba, Plasma Laser Process. Mater. [pap. conf.], Upadhya et al., eds., Miner. Met. Mater. Soc., [1991]).
U.S. Pat. No. 5,463,271 discloses a surface treatment for diamond that further lowers the vacuum energy level with respect to the electron energy level at the diamond surface. As is evident from FIG. 1, even a 1-electron-volt decrease in the work function of the emitter surface does not enable electrons at the deeper donor levels to be efficiently emitted at low applied voltages. Such a decrease would, however, permit emission of electrons from diamond with substitutional nitrogen donors into vacuum without barrier.
The historical development of field-emitter technology has nonetheless effected an implicit bias against this type of material on the basis of its high electrical resistivity--for example, type Ib diamond containing substitutional nitrogen donors to a concentration of about 10.sup.19 atoms cm.sup.-3 exhibits electrical resistivity levels greater than 10.sup.16 ohm-cm at low field strength. Previous cold cathode technologies have used materials such as metals or doped silicon as the emitter material. For such electrically conductive materials, the value of the electrical resistivity is an important figure of merit in the design of cold cathodes. However, for a wide-bandgap emitter material without shallow donors, such as diamond, the properties at the back contact transcend the bulk resistivity, especially at low field strengths; in other words, electron injection into the emitter material is the rate-limiting process. Yet despite the mediocre emission characteristics of higher-electrical-conductivity diamond compositions in cold-cathode applications, practitioners have continued to emphasize their use.
Improvements in the emission characteristics of diamond cold cathodes have been achieved through innovation in the geometry and chemistry of the diamond/vacuum interface. Unfortunately, scant research attention has been devoted to optimizing the features of the back contact with the diamond which are necessary for efficient electron injection, which would expand the range of materials--including those with highly favorable NEA properties--useful in the production of efficacious cold cathodes.